An accurate measurement of the pulsatile blood pressure in a person's cardiovascular system is a necessary component in most medical procedures. An individual's blood pressure can be represented in the form of an arterial waveform representing the instantaneous pressure continuously. Normally such a waveform can only be accurately measured with an intraarterial catheter and pressure transducer. The catheter can continuously monitor blood pressure, but suffers from the obvious drawback of requiring surgical intervention and associated risks of complications.
Many devices allow blood pressure to be measured by sphygmomanometer systems that utilize an inflatable cuff applied to a person's arm. The cuff is inflated to a pressure level high enough to completely occlude a major artery. When air is slowly released from the cuff, blood pressure can be estimated by detecting well-known "Korotkoff" sounds using a stethoscope or other detection means placed over the artery. A major drawback with these devices is that they can only provide an intermittent measure of blood pressure; and they do not provide a continuous arterial pressure waveform.
Some relatively recent inventions have made possible noninvasive methods of monitoring blood pressure. Some of these devices are also capable of detecting the arterial waveform. One such method is presented in U.S. Pat. No. 4,295,471 issued to William J. Kaspari. This patent discloses a sensor which uses two piezoelectric elements placed in a housing, and facing opposite directions. These piezoelements are acoustically and electrically isolated from each other. The sensor is placed in a pocket at the distal end of an inflatable cuff, which is then wrapped around a person's arm. The device disclosed in the '471 patent monitors the Korotkoff sounds as the pressure in the inflatable cuff is increased to a level just greater than that necessary to occlude the artery. One piezoelectric sensor element is placed directly over the artery and detects the Korotkoff frequencies, as well as motion artifact signals produced by muscle movement and/or other external factors. The second sensor element is placed directly against the air bag. Because the air bag highly attenuates the Korotkoff frequencies, the second sensor essentially only detects an artifact signal, which is substantially equivalent to the artifact portion of the signal detected by the first sensor element. The signal from the second sensor element is subtracted from the signal from the first sensor element, providing an arterial waveform with reduced common-mode artifact.
The '471 patent teaches detecting the arterial waveform in a noninvasive manner that mitigates artifactual contamination of the signal of interest. However, the device disclosed in the '471 patent is not intended to be disposable, and does not lend itself to application at body sites such as the radial artery, and generally requires a pressure cuff applied over the sensor. All other devices which externally measure the arterial waveform also suffer from the common problem of including a large artifact signal along with the arterial signal.